1. Field of the Invention
The present invention relates to a glass fiber sizing agent that is used for the surface treatment of glass fibers, which glass fibers are to be utilized for the manufacture of glass fiber reinforced plastics (sometimes hereinafter referred to as FRP).
2. Description of the Prior Art
Unsaturated polyester resins and epoxy resins are usually employed as thermosetting matrix resins in glass fiber reinforced plastics. The impregnating property, wetting property and adhesiveness of the matrix reins to glass fibers have great effects on the moldability of the glass fiber reinforced plastics and the quality of the final molded articles. The surface treatment of the glass fibers is very important for improving the impregnating property and the wetting property of the matrix resins.
In connection with the transparency of the final molded articles, the so-called opacity phenomenon readily occurs because of peeling of some of the glass fibers from the matrix resin and the molded articles often become opaque. Sometimes this opacity phenomenon is caused by a difference of the refractive index.
It is a primary object of the present invention to provide a glass fiber sizing agent capable of giving fiber reinforced plastics (FRP) wherein the glass fibers are bonded tightly to the matrix resin, occurrence of the opacity phenomenon is prevented, the impregnating and wetting properties of the matrix resin to the glass fibers are highly improved and yellowing does not take place at all.
The glass fiber sizing agents conventionally used in the art include polyvinyl acetate emulsions, aqueous solutions of polyvinyl alcohol and compositions containing a resin such as an unsaturated polyester or a saturated polyester, a lubricant and a polymerizable organic silane compound or a polymerizable organic chromium compound.
Polyvinyl acetate emulsions, aqueous solutions of polyvinyl alcohol and saturated polyesters are nonreactive to the glass fibers and are inferior in the glass fiber-bonding property. Accordingly, when these resin components are employed as sizing agents, the opacity phenomenon readily occurs. In contrast, when an unsaturated polyester resin is used as the resin component of the sizing agent, because this resin has reactivity with the glass fibers, the opacity phenomenon scarcely occurs. Especially, a sizing agent comprising as the main component an unsaturated polyester of the bisphenol type has excellent wetting and impregnating properties, in addition to the above advantage that the opacity phenomenon does not occur. However, a sizing agent of this type is defective because yellowing of the resulting FRP readily occurs.
Although the sizing agent of the present invention is of the saturated polyester type, because an alkylene oxide adduct of 2,2-bis(4-hydroxyphenyl)propane, i.e., an alkylene oxide of bisphenol A, is employed as the starting diol, by virtue of the inherent excellent mechanical properties of the bisphenol A structure, the opacity phenomenon does not occur at all in the resulting